Ambulatory infusion systems and devices for the infusion of a liquid drug over an extended time period are known in the art for a number of therapies. In particular, such devices form the basis for a state-of-the-art therapy for Diabetes Mellitus by CSII (Continuous Subcutaneous Insulin Infusion). A suitable device is disclosed, for example, in WO2003053498A2 to which reference is made for the general design and features of such devices.
Besides diabetes therapy, such infusion devices may be used for a number of other therapies, such as cancer treatment or pain therapy, without substantial modification. The following description mainly refers to CSII therapy as an example of one specific application. However, it should be understood that the embodiments described herein are not limited to this specific therapeutic application.
When using an ambulatory infusion device a number of error situations or hazardous situations may occur such as occlusions of the infusion cannula or the infusion tubing. Accordingly, it is desirable to detect such situations as early as possible. For this purpose, the fluidic pressure of the drug may be continuously monitored. In state-of-the-art devices this is typically done by measuring the reaction-force exerted by a plunger of the drive system of the infusion device on a plug of the drug cartridge. Corresponding designs are disclosed in U.S. Pat. No. 6,362,591, among others. This approach, however, involves a complex and critical electro-mechanical design of the infusion system, substantially adds to the size of the device and involves a number of measuring uncertainties due to friction. It is therefore favorable to directly measure the fluidic pressure of the drug. Pressure measurement arrangements such as those described in WO 2007/093064 have been designed for this purpose. Those arrangements, however, involve complex and expensive disposable components. Furthermore, it would be favorable to measure the drug pressure directly at the infusion cannula rather than at the pump outlet.
U.S. Pat. No. 6,830,558 B2 discloses a flow condition sensor assembly for an infusion device with a first electrode which is secured on a diaphragm and a second electrode which is positioned in a fixed location with respect to the first electrode. An impedance meter is connected to the electrodes. The diaphragm is resilient and may flex into a curved sensor chamber, thus modifying the impedance which may be measured between the first electrode and the second electrode. However, the sensor comprises complex components and must be carefully assembled.